The present invention relates to a charge/discharge system capable of supplying electric power to an electric motor generator and of charging with regenerative electric power from the electric motor generator.
Conventionally batteries have been used to supply electric power to an electric motor as a driving-power source for a vehicle or the like. Examples of the battery used include secondary batteries such as a lead storage battery and a lithium-ion battery. Other devices other than batteries also are known to charge/discharge electric energy using an electric double-layer capacitor or the like.
Batteries have large storable capacity, but their life will be shortened if charge/discharge is performed frequently or with a large charge/discharge depth. On the contrary, capacitors are robust over such frequent charge/discharge or a large charge/discharge depth and have a long life, but have relatively small storable capacity. In this way, batteries and capacitors are different in their characteristics, and therefore when a battery and a capacitor are mounted together, it is necessary to control them in accordance with their respective characteristics.
JP 2008-035670 A discloses an electric-power supplying apparatus for a vehicle provided with a plurality of charging mechanisms including a battery and a capacitor that is configured to keep the sufficient charging amount of the capacitor whose output performance is superior to the battery.
The electric-power supplying apparatus described in JP2008-035670A is configured to it the voltage of the capacitor is lower than the boosted voltage obtained by increasing the rated voltage by a boosting converter, charge the capacitor with electric power supplied from the battery. This may lead to the risk of deterioration in energy efficiency because charge discharge is performed between the battery and the capacitor.
In view of the aforementioned problems, it is desirable to improve energy efficiency of a charge/discharge system including a capacitor and a secondary battery.
According to one aspect of the present invention, a charge/discharge system capable of supplying electric power to an electric motor generator and of charging with regenerative electric power from the electric motor generator is provided, the charge/discharge system including: a capacitor connected to the electric motor generator; a secondary battery connected in parallel to the capacitor; an electric power converter placed between the capacitor and the secondary battery to increase/decrease charge/discharge voltage of the secondary battery; and a compeller to control the electric power converter. The controller includes: a request power calculation unit configured to calculate request input output power for the electric motor generator based on current and voltage of the capacitor and input/output current of the electric power converter; a capacitor discharging bias factor map configured to specify a ratio of electric power to be supplied from the capacitor to the electric motor generator to the request input power calculated by the request power calculation unit, based on the SOC of the capacitor; a capacitor charging bias factor map configured to specify a ratio of electric power to be stored in the capacitor from the electric motor generator to the request output power calculated by the request power calculation unit, based on the SOC of the capacitor; and a subtraction unit configured to calculate charge/discharge power of the secondary battery by subtracting, from the request input/output power, the charge/discharge power of the capacitor obtained from the capacitor discharging bias factor map or the capacitor charging bias factor map.